Surface treatment of metals



SURFACE TREATMENT OF METALS Thomas Gibson, Glasgow, Scotland, assignor to Metal- Gas Company Limited, Glasgow, Scotland No'Di-awing. Application December 21, 1953, SerialNo. 399,611

Claims priority, application Great Britain November 19,1948

4 Claims. (Cl. 11765) This invention relates to the surface treatment of metals 5 and more especially to theuse of surface coatings to improve the resistance to oxidation and corrosion at high temperatures of metal alloys.

Suchalloys may be composed of two or moreof the elements: aluminium, chromium, cobalt, columbium, copper, item-manganese, molybdenum, nickel, niobium, silicon, titanium, tungsten, tantalum, vanadium, zirconium and boron.

It is already known to produce a surface layer of high chromium content on such alloys. Thus, such'alloys are brought into contact at high temperatures with agas having as a constituent a chromium compound such as chromous chloride. The chromium is deposited-on the surface of the alloy by an interchange reaction and diffuses inwards from the surface of the article being-treated.- By 0 this means a surface Zone is obtained which contains a high percentage of chromium-and which is so alloyed to the base material as to be an'integi-al part of it. 7 Such a process is an example of chromising which when used herein means diifusingchromiuiii into the surface of an article by any method in which the article is brought into intimate contact with gaseous chromium or a gaseous chromium compound at an elevated temperature. This process is to be contrasted with processes such as chromium electroplating, metal spraying and cathode splutter- 40 ing which, although directly or as a result of an after heat treatment, might also give rise to inward diffusion of chromium, always leave a surface layer consisting essentially of chromium. Chromising' is distinguished from these processes by the critical feature that no pure chromium surface is formed. The alloy'surface formed as a result of chromising rarely contains more than about of chromium because the process is based on an interchange reaction which becomes excessively slow as the surface content of metal other than chromium diminishes and the depth of the chromium-containing surface layer increases.

Such chromising treatment has been found to increase the resistance to oxidation and corrosion at high temperature of certain of these alloys. in other cases, however, the anticipated increase of resistance to oxidation and corrosion does not occur.

it is an object of this invention to increase the protection against oxidation and corrosion of chromised alloys at high temperatures.

It is a further object to reduce the rate of scaling at high temperatures of such alloys.

According to the present invention after an article of an alloy of the kind described has been chromised, a superficial layer of aluminium is applied to the surface of the article. When this layer is oxidised and alloyed with the base material by heating in an oxidising gas such as air or in some other oxidising medium, greatly improved resistance at high temperature is obtained.

It is not known exactly why this treatment produces such striking results.

The chromising may be carried out in any one of 2,809,127 Patented Oct. 8, 1957 ice several known ways but the method used is not material to the final result which is obtained no matter in what way the chromium-rich surface layer has been-obtained.

The aluminium may be applied by any convenient method such as metal-spr'aying; or by the process of calorizing in which the article is heated in contact with powdered aluminium and aluminium oxide; or an aluminium-containing paint or powder may be applied to the surface with the help of a bonding agent.

After coating with aluminium by any of these means, and the removal of any moisture, the article is heated to form the oxide film and to cause the aluminium to diffuse into and alloy with the surface layer of the coated article.

A temperature of about 800 C. for 30 minutes is suitablethough this does not appear to be critical.

In many cases all that is required is to put the article into service since the operating conditions are themselves sufficient to effect the necessary surface stabilisation by oxidation.

The following examples are given by way of illustration to demonstrate the improved resistance afforded by the process:

EXAMPLE 1 A piece of commercial heat-resisting alloy containing chromium, cobalt, iron, molybdenum and nickel was chromised by heating it in an atmosphere containing chromous chloride for six hours at 1050 C. It was then coated with a surface layer of aluminium paint having an oil base. The paint was allowed to dry and the treated specimen was placed in a heating oven along with an untreated specimen of the same alloy. The two specimens were heated in air at 1150 C. for eight hours. At the end of this period the untreated specimen had acquired a heavy, black oxide scale about fla thick which flaked off easily on cooling. appreciable oxide scale whatever and had only slight dis-coloration.

EXAMPLE 2 A piece of commercial heat resisting alloy containing nickel and 20% chromium was chromised in similar manner to Example 1 and was then sprayed with a thin film of aluminium metal. This specimen was heated to 1200 C. in air alon with another piece of the same material in the untreated condition. After 24 hours heating both specimens were removed from the furnace for examination. The untreated specimen was coated with a thick, poorly adherent, powdery oxide scale, Whereas the treated specimen had a thin firmly adherent oxide scale.

Further comparative experiments were caused to be made, using an apparatus consisting of a balance from which was suspended the specimen under test, the specimen hanging in the centre of a nichrome wound furnace. A thermocouple was also placed in the furnace and connected to a Cambridge potentiometric controller so that the tip of the thermocouple and therefore the control point coincides with the centre of the suspended specimen. The balance was fitted with a rider attachment so that a weight variation of 1.6 gms. could be determined from outside the case. The balance was supported on a rigid The treated specimen exhibited no 250 grams per litre of chromium oxide and 22.5 grams per litre of sulphuric acid at 53 C. for 2 hours with a current density of 300 amps. per square foot. This chromium plating process is equivalent to that disclosed by U. S. Patent No. 1,746,987 of Bennett, at page 1 line 89 to page 2 line 2.

B. Chromium plating followed by difiusin.Specimens which have been chromium plated as described in A above were heated in an atmosphere of hydrogen for 5 hours at 1,000 C. to enable diffusion of the plated layer to occur.

C. Chr0mising.Steel specimens were chromised in a retort for 6 hours at a temperature of 1,050 C. The amount of chromium taken up was rather less than the amount deposited on the plated samples.

In carrying out the tests, the exact measurements of each specimen were obtained then the specimens were supported in a cradle of platinum wire and dipped into bituminous aluminium paint. The specimens were allowed to dry and were ignited for 30 minutes at 800 C. They were then attached to the suspending wire and lowered into the furnace. Care was taken during this operation not to cause flaking of the aluminium layer. As the furnace was already at the testing temperature, the initial weighing was taken. Subsequent weighings were taken at intervals during the duration of the test.

To test the reproduceability of the apparatus and experimental technique, two short runs of 24 hours were carried out, using chrome plated and diffused specimens at a temperature of 900 C. The following results were obtained:

The above results giving a variation of 2 in 75 would seem to indicate a satisfactory degree of accuracy.

A complete run of 100 hours duration was then carried out at temperatures of 900 C. and 950 C. on each type of specimen. The following results were obtained:

Scaling Index (mgms/emfi) 900 C. 950 C. Time Cr. Cr. Chro- Cr. Cr. Chro- Plated Plated mised Plated Plated mised Difi. Difi.

The figures under the heading Scaling Index represent a measure of the amount of scale formed at the stated temperature and during the stated periods of time, for each of the three types of specimen.

The surface of the specimens were examined after each run. In every case this showed a thin coating of white aluminous material with a green scale beneath. Both layers were thin and easily removed leaving a green surface skin of oxide underneath which firmly adhered to the specimen.

It is apparent from these tests that a chromising followed by painting with aluminium gives better protection against oxidation at high temperatures than does chr0- mium plating followed by aluminium painting, whether or not the chromium plating is caused partly to diffuse into the heat before the aluminium painting is carried out. Chromised and aluminium painted specimens, heated at 900 C. lost only half as much weight as the chromium plated and painted specimens.

I claim:

1. In the treatment of metal alloys to increase their resistance to oxidation and corrosion at high temperatures, the process which consists substantially in first chromizing the alloy to be treated; said alloy being composed of at least two metals selected from the class consisting of aluminium, chromium, cobalt, copper, iron, manganese, molybdenum, nickel, niobium, silicon, titanium, tungsten, tantalum, vanadium, zirconium and boron; to produce a surface layer enriched in chromium but not composed entirely of this metal; then applying a superficial coating of aluminium and heating in an oxygen-containing atmosphere to oxidize the aluminium coating and to alloy it with the chromized surface of the said alloy.

2. A process as set forth in claim 1 in which the oxidising is carried out by heating in air at about 800 C. for about 30 minutes.

3. A process as set forth in claim 1 in which the metal is a chromium-cobalt-nickel-molybdenum alloy steel.

4. A process as set forth in claim 1 in which the metal is a nickel-chromium alloy containing about 20% chromium and nickel.

References Cited in the file of this patent UNITED STATES PATENTS 2,141,640 Cooper Dec. 27, 1938 2,157,594 Cooper May 9, 1939 2,255,482 Daeves et al. Sept. 9, 1941 2,536,774 Samuel Jan. 2, 1951 2,657,127 Sindeband et al. Oct. 27, 1953 FOREIGN PATENTS 698,897 Germany Nov. 19, 1940 

1. IN THE TREATMENT OF METAL ALLOYS TO INCREASE THEIR RESISTANCE TO OXIDATION AND CORROSION AT HIGH TEMPERATURES, THE PROCESS WHICH CONSISTS SUBSTANTIALLY IN FIRST CHROMIZING THE ALLOY TO BE TREATED; SAID ALLOY BEING COMPOSED OF AT LEAST TWO METALS SELECTED FROM THE CLASS CONSISTING OF ALUMINIUM, CHROMIUM, COBALT, COPPER, IRON, MANGANESE, MOLYBDENUM, NICKEL, NIOBIUM, SILICON, TITANIUM, TUNGSTEN, TANTALUM, VANADIUM, ZICONIUM AND BORON; TO PRODUCE A SURFACE LAYER ENRICHED IN CHROMIUM BUT NOT COMPOSED ENTIRELY OF THIS METAL; THEN APPLYING A SUPERFICAL COATING OF ALUMINIUM AND HEATING IN AN OXYGEN-CONTAINING ATMOSPHERE TO OXIDIZE THE ALUMINIUM COATING AND TO ALLOY IT WITH THE CHROMIZED SURFACE OF THE SAID ALLOY. 